In related-art image forming apparatuses that form an image on a sheet of recording media according to image data, a sheet feeding device incorporated load a plurality of sheets and feed the sheets one by one toward an image forming device. The image forming device then forms an image on a sheet supplied from the sheet feeding device. Such a device encounters the problem of how to cleanly separate the sheets for forwarding to the image forming part, for which various solutions have been advanced.
As one approach, an electrostatic sheet feeding method to separate and feed a sheet electrostatically has been proposed, for example, in Japanese Patent No. 3159727. In the electrostatic sheet feeding method, a dielectric endless belt is wound around multiple rollers and a charging member applies alternating electrical charges to the surface of the dielectric endless belt. The thus-charged dielectric endless belt then contacts or nearly contacts a stack of sheets so as to attract an uppermost sheet of the stack of sheets thereto. After a predetermined period of time, the dielectric endless belt is physically separated from the stack of sheets and then is rotated to feed and convey the uppermost sheet attracted to the dielectric endless belt forward.
When the leading edge of a sheet of the stack of sheets is curled or burred by cutting, the sheet is likely to separate from the dielectric endless belt too easily, which can cause sheet conveyance failure. More specifically, the attractive force of the dielectric endless belt is storing with respect to a force exerted in a sheet conveyance direction but weak with respect to a force exerted in a vertical direction perpendicular to the belt surface. With the leading edge of the stack of sheets curled or burred by cutting, the adhesive force exerted at the leading edge of the sheet in the vertical direction perpendicular to the belt surface may increase. Therefore, when the dielectric endless belt is separated from the stack of sheets, the uppermost sheet can separate too easily from the dielectric endless belt due to the relative weakness of the adhesive force at the leading edge of the stack of sheets, therefore producing sheet conveyance failure.
Similar problems can occur when using an air-driven sheet separation method, in which air is blown from a direction opposite an edge surface of a moving direction of a surface of a stack of sheets to separate an uppermost sheet from the other sheets and then attract the uppermost sheet to the dielectric endless belt.
As another approach, Japanese Patent Application Publication No. JP 2003-160248 (JP-2003-160248-A1) discloses a sheet feeding device that includes a first attraction belt disposed upstream from the leading edge of the stack of sheet in the sheet conveyance direction and a second attraction belt disposed facing the leading edge of the stack of sheets and having an attractive force stronger than that of the first attraction belt. Since the second attraction belt having a stronger attractive force than the first attraction belt attracts the leading edge of the sheet, even when the attractive force between the sheets at the leading edge thereof is strong due to curl or burr formed by cutting at the leading edge of sheets of the stack, the leading edge of the uppermost sheet can be separated from the stack of sheets.
However, in JP-2003-160248-A1, an attraction unit including the second attraction belt (e.g., a unit composed of the second attraction belt, multiple tension rollers around which the second attraction belt is wound with tension, an applicator to apply an attractive force to the second attraction belt, and a contact and separation drive unit to drive the second attraction belt to contact or separate from the stack of sheets) is disposed separately from an attraction unit including the first attraction belt. Accordingly, installation of separate attraction units or components can increase costs of an image forming apparatus.